iStock/Thinkstock(WASHINGTON) — When President-elect Donald Trump takes office on Jan. 20, one of the most immediate and critical decisions he’ll have to make is filling the vacancy left on the Supreme Court by the death of Justice Antonin Scalia.

President Obama nominated Judge Merrick Garland earlier this year, but the nomination will expire before Obama leaves office in January if he is not confirmed, which is likely.

Trump — whose own position on abortion has shifted over time — told CBS News’ 60 Minutes last week he was “pro-life” and would appoint individuals that held the same position to the nation’s highest court. Speaking hypothetically, the president-elect said, “If it ever were overturned, it would go back to the states.”

While the addition of one conservative justice is unlikely to realign the nation’s highest court in such a way that the landmark abortion rights decision Roe v. Wade could be threatened, the replacement of additional justices, could, according to Kate Shaw, an ABC News contributor and Cardozo law professor, have major consequences. We asked Shaw to help explain how this might work.1]What is Roe v. Wade and what did it do?

KS: In Roe v. Wade, the Supreme Court held that the Constitution protects a woman’s right to decide whether to terminate a pregnancy. At the time Roe was decided, 1973, many states criminalized abortion, and Roe struck all of those criminal statutes down. Following the decision, states weren’t able to prohibit abortion outright; but they do have the power to regulate it.2] What can states do to regulate abortion?

KS: Roe held that while the Constitution protects a woman’s right to decide for herself whether to continue with a pregnancy, states also have a strong interest in regulating abortion, both to protect women’s health and to promote potential life.

The Court balanced these competing interests using a trimester framework, holding that during the first trimester of pregnancy, the abortion decision is in the hands of a woman and her doctor. During the second trimester, the state can regulate abortion in ways that are reasonably related to women’s health. And in the third trimester, the state’s interest in potential life is strong enough to allow states to prohibit abortion outright, except where necessary to preserve a woman’s life or health.

In a 1992 case called Planned Parenthood v. Casey, the Court abandoned the trimester framework and held that states have a valid regulatory interest for the duration of a pregnancy. But it also held that prior to viability, states may regulate abortion only so long as the regulations do not impose an “undue burden” on a woman’s right to choose. Under Casey, states have regulated abortion in a range of ways, from requiring counseling about the availability of adoption to mandatory waiting periods to the regulation of facilities that perform abortions. Earlier this year, in a case out of Texas, the Supreme Court made clear that where states act to regulate abortion facilities, those regulations must be grounded in objective medical standards.

3] Let’s assume Trump appoints a person ideologically in line with Scalia. What will be the court’s ideological makeup at that point?KS: Justice Scalia was a strong opponent of Roe and Casey; a jurist who shared his views in this area would maintain the status quo, which is four strong votes in favor of Roe, four Justices who don’t believe the Constitution protects the right to abortion, and Justice Kennedy, who has voted with both blocks when it comes to abortion (though it’s significant that his most recent vote in an abortion case was with the liberals). So statutes like the Texas clinic regulation, which Kennedy joined the liberals in striking down, would remain invalid.

4] Let’s assume liberal Justice Ruth Bader Ginsburg and swing vote Anthony Kennedy retire, leaving open two slots. And then Trump appoints two more conservative justices. How precisely would the court be able to overturn Roe v Wade. What needs to happen?

KS: There are a couple of ways this could happen. First, a legislature could pass either an outright abortion ban or an extremely restrictive abortion regulation, either of which could set up a test case in which the Supreme Court would be invited to overrule Roe.

It could also happen more gradually, and I think that’s more likely, as the Court tends to prefer to act incrementally; the Court could uphold a series of abortion regulations, gradually weakening the foundations of Roe and Casey and then finally overruling those cases.

But — and this is a big but — there’s no absolute guarantee that Roe would be overruled, even with three or more Trump appointees. It’s at least possible that the Court would decide to adhere to Roe on the grounds that sometimes “it is more important that the applicable rule of law be settled than that it be settled right.” Even a conservative Court could decide it would simply be too disruptive to overrule this long-settled precedent on such a divisive social issue. That’s essentially what happened in Casey, when everyone thought Roe would be overruled — but it wasn’t.5] Would that ruling have immediate nationwide impact?

KS: A decision overruling Roe would allow states to ban abortion, but wouldn’t require them to do so. And the big question would be which states would do that. A number of states have laws on the books that would essentially ban abortion immediately if Roe were overruled; other states would likely move quickly to pass such laws. In all, over half of the states would likely eliminate or severely restrict access to abortion within their borders if Roe were overruled.